4 years ago

Noncovalent Control of the Electrostatic Potential of Quantum Dots through the Formation of Interfacial Ion Pairs

Noncovalent Control of the Electrostatic Potential of Quantum Dots through the Formation of Interfacial Ion Pairs
Monica Olvera de la Cruz, Trung D. Nguyen, Chen He, Kedy Edme, Emily A. Weiss
This paper describes the role of tetraalkylammonium counterions [NR4+, R = −CH3, −CH2CH3, −(CH2)2CH3, or −(CH2)3CH3] in gating the electrostatic potential at the interface between the 6-mercaptohexanoate (MHA) ligand shell of a PbS quantum dot (QD) and water. The permeability of this ligand shell to a negatively charged anthraquinone derivative (AQ), measured from the yield of electron transfer (eT) from the QD core to AQ, increases as the steric bulk of NR4+ increases (for a given concentration of NR4+). This result indicates that bulkier counterions screen repulsive interactions at the ligand/solvent interface more effectively than smaller counterions. Free energy scaling analysis and molecular dynamics simulations suggest that ion pairing between the ligand shell of the QD and NR4+ results from a combination of electrostatic and van der Waals components, and that the van der Waals interaction promotes ion pairing with longer-chain counterions and more effective screening. This work provides molecular-level details that dictate a nanoparticle’s electrostatic potential and demonstrates the sensitivity of the yield of photoinduced charge transfer between a QD and a molecular probe to even low-affinity binding events at the QD/solvent interface.

Publisher URL: http://dx.doi.org/10.1021/jacs.7b05501

DOI: 10.1021/jacs.7b05501

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